In ball games played with such rackets, a ball having a given mass is struck to impart a desired acceleration and direction to the ball. This is mainly due to an impact so that the transmission of momentum is of decisive significance. The forces required for this purpose are derived from the kinetic energy of the moving racket and from the muscle force of the player. The contribution of the kinetic energy depends on the mass or moment of inertia of the racket and on its angular velocity at the instant of the impact. At a given velocity, a high moment of inertia or a large mass will produce a large momentum and impart a high acceleration to the ball. On the other hand, a larger effort is required to accelerate a heavier racket. When the racket is too light, it can be more highly accelerated but a larger share of the momentum of the impact must be produced by the player. The deformation of the stringing, frame, and handle will depend on the stresses which are produced and on the stiffness of the material.
Different motions can be imparted to the ball in a game by a variation of the stiffnesses and the mass or the moment of inertia.
The previously known rackets consist of wood, metal or plastics material alone or in combination. Each type has a number of advantages and disadvantages which are specific thereto and which relate to the properties of the racket or to its manufacture. Almost all known types have the disadvantage that the properties cannot be sufficiently varied owing to considerations as regards the weight, the required strength properties, or the process of manufacturing the racket.
It has already been attempted to avoid these disadvantages by the production of a tennis racket which comprises eight rovings, which have been cured in a mold around an inflatable tube, by which the required pressure is applied.
In rackets having such a frame, the tube formed two legs extending in the handle and adjacent to the gusset at the transition from the handle into the oval frame diverged into the latter. Thus resulted in the disadvantage that no pressure was applied by the tube to that edge portion of the gusset which faced the stringing. The resulting racket frame was provided with a pregreg covering only in the lower one-third portion of the oval frame, next to the handle, as well as adjacent to the gusset and around the handle whereas the rovings were not covered in the remaining portion of the frame. As a result, the tube can hardly extend in the frame at the center of the cross section because it can yield in an uncontrolled manner inwardly and outwardly to the outer contour. This results in irregular cross-sections and in irregular mechanical properties of the frame.
Finally, laminated tennis rackets have been disclosed, which comprise a foam core which generally conforms to the contour of the tennis racket and at its surfaces which are parallel to the stringing is covered by metal or plastics materials skins having a yield strength above 3500 kilograms per square centimeter and a Young's modulus above 70,000 kilograms per square centimeter, and at its surfaces which are at right angles to the stringing is covered by straps having a compressive strength of about 315-1410 kilograms per square centimeter and a Young's modulus of about 70,000 kilograms per square centimeter and consisting preferably of polyethylene. These straps serve to strengthen the surfaces covered by them. This racket has the disadvantage that the stringing tends to slacken as it cuts into the foam core, which is protected only by a plastics material strap having a low stiffness. Besides, the metal plates must be cut from relatively large plates so that there is a substantial amount of waste.